The classical form of a-1 antitrypsin deficiency (ATD) is caused by the expression of a misfolded mutant a-1 antitrypsin (ATZ). ATD is the most common genetic cause of liver disease in children and is associated with increased risk for chronic liver disease, hepatocellular carcinoma, and pulmonary emphysema. One of the most important characteristics of liver disease in ATD is that it is highly variable in severity and therefore modifiers (genetic, environmental) have a major impact on the hepatic phenotype. Available models for ATD have been useful for elucidating many aspects of the disease and, more recently, in the development of novel therapeutic candidates for ATD patients. We recently showed that the stress of expression and/or accumulation of misfolded ATZ in the livers of PiZ mice, a model of ATD, provided a competitive advantage to transplanted wildtype hepatocytes, leading to progressive repopulation ofthe host liver. These results indicate that hepatocyte transplantation may be effective therapy for a subset of patients with ATD. In this application we propose 2 series of experiments to further investigate the selective proliferative advantage of transplanted hepatocytes in the PiZ mouse model of ATD. First, we will investigate the extent to which human hepatocytes can re-populate the liver using a strain of PiZ mice that have been bred onto an immune deficient background (PiZ-SCID). Second, we will determine whether human IPS cells from normal individuals that are differentiated into hepatocytes (iPS-derived hepatocytes) can repopulate the liver ofthe PiZ-SCID mouse. In the last part of this proposal we will exploit a mouse model that gives transplanted human hepatocytes a selective proliferative advantage over host hepatocytes to generate 'humanized' mouse models of ATD. iPS-derived hepatocytes from patients with ATD will be transplanted into the livers of immune deficient fumaryl acetoacetate hydrolase-deficient (Fah-/- Rag2-/- 7IL2rg-/-) mice, a mouse model which has been ideally adapted for re-population studies with human cells. These 'humanized' mouse models will provide two advantages over existing PiZ-based mice: putative therapeutic drugs can be tested pre-clinically on human cells in vivo and therein uncover potential differences in efficacy or toxicity between murine and human cells; pathobiological and therapeutic studies can be carried out in animal models of ATD that also bear patient-specific modifiers. These models will be an ideal complement to the studies that will be carried from the C. elegans model in Project 2 to mammalian cell line and existing mouse models in Project 1 to take us into the era of personalized medicine for treatment of ATD.